A Tibetan monk who was one of China's most prominent political prisoners has died in jail, a relative said on Monday, and a rights group said security forces were deployed after protesters gathered near a government office in his home region.

The United States, the European Union and international rights groups had called for the release of Tenzin Delek Rinpoche, 65, who was serving a 20-year sentence on charges of "crimes of terror and incitement of separatism."

He died Sunday in Chuandong prison in the southwestern city of Chengdu, his cousin Geshe Nyima told Reuters by telephone from India.

Death confirmed

The Tibetan government-in-exile, the Central Tibetan Administration, also confirmed his death.

"The fact that he was not even allowed medical parole and last wish of followers to see him reflects continuing hardline policies of the Chinese government," said Lobsang Sangay, who heads the India-based administration. "Such mistreatment will only generate more resentment among Tibetans."

FILE - An exile Tibetan Buddhist monk holds a banner with the portrait of Buddhist leader Tenzin Delek Rinpoche who has been in a Chinese prison since 2002 on charges of “inciting separatism”, in Dharmsala, India, April 7, 2015.

The U.S. State Department said it was saddened to learn that Tenzin Delek had died.

"We hope Chinese authorities will investigate and make public the circumstances surrounding his death," it said in a statement on Monday.

Rights group Free Tibet said security forces had been deployed in Tenzin Delek's home county in China's Sichuan Province, where thousands of Tibetans had gathered outside a government office near his monastery.

Reuters could not independently verify the protest and security response. Authorities in the area were not available for comment.

Heart ailment

The cause of Tenzin Delek's death was not clear, his cousin said, but rights groups had said he was suffering from a heart condition.

Authorities notified his sisters about his death on Sunday, the cousin said.